1. Field of the Invention
The present invention relates to touch operated keyboards and more particularly to touch operated keyboards which may be operated through an unknown thickness of glass.
2. Description of the Related Art
In this specification the term glass is used since the keyboard will normally be operated through clear glass. The term glass is however to be interpreted to include any insulating material whether clear or opaque. With an opaque insulating material it is normally necessary to have an external faceplate to identify the touch area for each key. In other respects the problems of coping with various thicknesses of glass are the same whether it is clear or opaque.
GB Patent Application No. 2011089 (equivalent to U.S. Pat. No. 4,145,748 and DE. No. 2854539) shows a touch pad sensor circuit which is self optimising. In this system the pads are addressed by respective drive pulses to give output values, the previous "no touch" value of each pad being stored in a memory for comparison with each new value. Thus only when the new value exceeds the old by a predetermined amount will the circuitry decide that a key has been operated. This system suffers from the disadvantage that if a large capacitive object suddenly comes close to all the keys then the decision logic circuitry will conclude that all keys have been operated. Additionally the system is not sensitive enough to operate through very thick glass as found in shop windows because the thick glass produces only a small capacitive change when touched. Thus whilst this known system is acceptable in a home environment on for example kitchen ranges (see page 1 lines 8-14) it is not practical for shop windows where the glass is thicker and where large capacitive disturbances are possible.